The GSM-network, common in Europe, is a mobile radio network that uses time division multiple access (TDMA). As with other mobile radio networks, the GSM network employs authorization checks and encryption of transmitted messages. With regard to the GSM network, this is specified in "GSM specification 03.20", May 1994, issued by ETSI (European Telecommunication Standard Institute) and hereinafter referred to as ETSI/GSM 03.20. The various algorithms used in authorization checks and encryption are described in this reference.
As described in ETSI/GSM 03.20, an algorithm A3 is used to effect actual authorization checks between network and subscriber apparatus. As further described, an algorithm A5 is used for encryption of the payload information to be transmitted, and an algorithm A8 is used to form, from the subscriber authorization key Ki, an encryption key Kc.
As a rule, only one time slot per frame for a given connection is used in TDMA-type time division mobile radio systems; see ETSI/GSM 05.02.
The use of two or more time slots, not necessarily consecutive time slots, in a transmission frame has been proposed, see ETSI/STC SMG3, T doc SMG3 WPA 95A dated Aug. 29, 1995 (Nokia Telecommunications), see particularly point 5 "HSCSD Architecture". This provides the advantage of enabling larger quantities of information to be transmitted per unit of time (applicable particularly to data transmissions), but has the drawback of increasing bandwidth.